The present invention relates to the identification of thermally induced turbulence in fluids, i.e. liquids, gases or vapours, and is particularly applicable to the detection of temporal variations in the temperature of flames.
The flow of a fluid with gradients of temperature may be measured in various ways, such as by thermocouples placed within the fluid. Such invasive methods may disturb the flow of the fluid itself and may be difficult to implement if the fluid is difficult to access or fills a large volume. Therefore remote sensing methods in which the fluid is imaged onto a detector array have advantages, though it is then necessary to devise methods for distinguishing the bulk temperature of the fluid from variations in emissivity or from the background.
An important class of fluid flow is that which occurs in flames. Combustion products within and above the envelope of a flame, such as carbon dioxide and water, are known to emit characteristic infrared radiation. It is also known that this radiation is not constant in time but varies (flickers) giving frequency components substantially between 1 Hz and 20 Hz. Known infra-red detectors isolate these wavelengths by means of a suitable spectral filter or use electronic signal processing of the detector output to detect this xe2x80x9cfilckerxe2x80x9d. In some cases additional sensors are used at different wavelengths in order to differentiate between flames and other sources of infrared radiation such as the sun, lighting equipment or hot machinery such as welders. Instruments of this type work well but cannot provide directional or spatial information because they consist of single element detectors looking into a wide viewing angle without imaging optics. Optical constraints may also give rise to high costs.
Sometimes spatial information is essential: for example if it were necessary to monitor two flames, one wanted and one unwanted in close proximity, or if the location of the flame within a protected area were required in order to selectively deploy countermeasures. Spatial data about the flame itself and its surroundings give the possibility of greater certainty of detection and a lower false alarm rate. In cases such as these an array of detectors may be used in conjunction with a mirror or infra-red transmitting lens which image the scene onto the array. The derived image may be analysed by computer system or monitored by eye. These instruments can provide a great deal more information about the scene viewed and in particular it is possible to discern structure within the flame itself.
The present invention was devised with the aim of accurately distinguishing flames from other hot objects emitting infrared radiation. The invention is based on the discovery that flames (even steady flames with no xe2x80x9cflickerxe2x80x9d exhibit distinctive temporal variations in temperature which, with the advent of array based detectors, can be identified. Nevertheless, the method of the invention has wider applications for identifying turbulence in general.
Thus, the present invention provides a method of identifying thermally induced turbulence in fluids comprising:
(a) forming an image of the fluid on an array of thermal detector elements;
(b) detecting thermal emissions from the fluid using the array; and
(c) repeatedly examining the relationship between the thermal emission received by a first element at a particular point in time and the thermal emission received by a second element at a subsequent time whereby to detect temporal variations in temperature due to turbulence.
It has been found that in a flame a xe2x80x9chot spotxe2x80x9d in the absence of draughts, tends to progress upwardly. Therefore, as a minimum, the method of the invention will examine two elements, the second element corresponding to a portion of the fluid which is higher than that to which the first element corresponds. Usually the two portions of the fluid will be adjacent to each other. In order to examine the whole of the flame structure, the examination may be repeated for all vertically adjacent pairs of elements in the array. Alternatively, the method may be confined to a portion of the array which has been identified as possibly viewing a flame by some other method. For example, the method of the invention may be confined to elements of the array which have been previously identified as exhibiting signals above a certain threshold, and possibly a set of elements surrounding those elements.
The method need not be confined to vertically adjacent pairs of elements and may be repeated for all adjacent pairs of elements in the array. For example, if a flame is affected by side winds, the turbulence trends may be side-ward rather than upward. Furthermore strong turbulence, which may arise in large uncontrolled flames, also leads to increasing correlation in all directions.
In the preferred embodiment of the invention, the relationship between signals from pairs of elements is examined by calculating the cross correlation function       c    ⁡          (      T      )        =            ∑      i        ⁢                  x        ⁡                  (                      t            i                    )                    ⁢              y        ⁡                  (                                    t              i                        +            T                    )                    ⁢              xe2x80x83            ⁢      for      ⁢              xe2x80x83            ⁢      different      ⁢              xe2x80x83            ⁢      values      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      T      
where i is an integer, x(ti) is the signal received from the first element at time (ti) and y (ti+T) is the signal received from the second element at time (ti+T); whereby to identify a maximum in the relationship between c(T) and T.
The significance of this mathematical relationship will be discussed in more detail below. The maximum value of c(T) may be compared with pre-set limits as can the value of T at the maximum value of c(T) as further steps in the correct identification of flames or other known phenomena.
The method described above will be best matched to low to medium resolution thermal infrared arrays. Typically the array will have at least 10 and not more than 10,000 elements and preferably at least 64 and not more than 1,024 elements.
Preferably only radiation at wavelengths longer than 2 micrometers is detected. Preferably the maximum wavelength radiation detected is 15 micrometers.
The invention also provides apparatus for carrying out the methods described above.